Most modern vehicles include a vehicle radio that requires an antenna system to receive amplitude modulation (AM) and frequency modulation (FM) broadcasts from various radio stations. In the United States, the FM transmission band is in the frequency range of 88 MHz to 108 MHz, and the AM transmission band is in the frequency range of 530 kHz to 1710 kHz. Many present day vehicle antenna systems include a mast antenna that extends from a vehicle fender, vehicle roof, or some applicable location on the vehicle. Although mast antennas provide acceptable AM and FM reception, it has been recognized by vehicle manufacturers that the performance of a mast antenna cannot be significantly increased, and therefore, improvements obtained in other areas of in-vehicle entertainment systems will not include reception capabilities of the mast antenna. Consequently, vehicle manufacturers have sought other types of antenna designs to keep pace with consumer demands for increased vehicle stereo and radio capabilities.
Improvements in vehicle antenna systems have included the development of backlite antenna systems, where antenna elements are formed on a rear window of the vehicle. The antenna elements are typically made of a conductive frit material deposited and patterned on the inside surface of the window. Backlite antenna systems provide a number of other advantages over mast antenna systems, including no wind noise, reduce drag on the vehicle, elimination of corrosion of the antenna, no performance change with time, limited risk of vandalism, and reduced cost and installation.
Most vehicles include conductive defogger elements on the rear window of the vehicle, also formed of the conductive frit material, that are electrically energized to heat the window to eliminate condensation and ice. It has heretofore been known in the art to use the same defogger elements as the antenna elements to provide AM and FM reception. Unfortunately, the voltage applied to the defogger elements causes noise interference to the AM reception that significantly affects antenna performance. This noise interference can also affect FM reception. To reduce the noise interference in the antenna backlite designs, it is known to provide antenna elements in the back window of the vehicle that are separate from the defogger elements.
Some of these designs use the separated antenna elements for both AM and FM reception. In this type of design, the antenna elements and the defogger elements are not directly connected to each other, but are spaced from each other so that defogger elements are coupled to the antenna elements and are driven as a parasitic antenna element. Since the defogger elements cover most of the viewing area of the rear window, the antenna elements are confined to an upper portion of the window. The vehicle body acts as a ground plane and is coupled to the antenna elements through a urethane seal that seals the rear window to a vehicle body flange. It is important to control the smallest distance between the antenna elements and the body metal ground plane to control the antenna impedance.
Other antenna designs use the rear window defogger elements for FM reception but separate antenna elements for AM reception. For these backlite antenna systems, FM energy is still received by the AM antenna elements. These FM signals can be coupled into the conductive vehicle body from the AM antenna elements and then from the vehicle body into the FM antenna. Also, FM coupling occurs between the AM antenna elements and the defogger elements. This FM signal coupling creates an impedance mismatch between the FM antenna feed and the RF amplifier that affects FM antenna performance. Therefore, it is desirable to provide additional impedance matching elements in connection with the defogger elements and the antenna elements in this type of backlite antenna system. It is an object of the present invention to provide such impedance matching elements.